This invention is particularly applicable to the field of electrical discharge machining in which a tool electrode is opposed to an electrically conductive workpiece to form a gap through which a dialectric fluid is passed. A power supply is included to provide machining power pulses across the gap so that particles of the workpiece are removed responsive to each gap breakdown. The pulses are provided with controllable "on-off" time and frequency to control the overcut and finish of the operation. As material is removed from the workpiece, a servo feed system is employed to advance the electrode toward the workpiece and continue the machining operation. In electrical discharge machining, sometimes hereinafter referred to as EDM, pulses are passed at high frequency across the machining gap with provision for selectively adjusting both on or off time or frequency as these changes are desired. At some points of operation, failures occur in the power supply, particularly in the output stage or power switches of the power supply. It is necessary to monitor the condition of these power transistors to determine whether transistor or other component failures have occurred. In this manner, it is possible to replace the failed components so that the power supply will always be operating at the optimum efficiency and with predictable results. The present invention is one that checks the power transistors without reliance on any timimg elements. Instead it checks the power transistors during the "off" time and if there is a failure, a reliable indication of that condition is provided.
The present invention not only detects a transistor failure but displays where that transistor is with an LED light. If any transistor turns on during normal "off" time, there will be a signal and the output will go to ground. An indication that the transistor is continuing to conduct during the "off" time provides a reliable proof that the transistor or something in the drive line has failed. An indicator light can be mounted on the machine to show when this condition has occurred so the corrective replacement can be made.